Sunday, April 3, 2011

Some of you have probably heard about the EU proposal that plans to end the internet as we know it on May 25th 2011. If you haven’t heard of it, David Naylor has made a nice little example of it’s consequences here. In essence most sites that use cookies will have to ask visitors to opt-in for every single cookie before using it. I’m very much in favor of online privacy – yet it seems to me that this is a very poorly thought through directive. First of all, most cookies server 1 of 2 purposes:

Help web sites recognize visitors in order to provide them with the best possible service. Much like when I walk into my local barbershop and the barber recognizes me and knows exactly how I prefer him to cut my hair (the little I have left after reading crazy directives) – and which subjects I want to small talk about.

Visitor tracking in order to do statistics the site owners can use to improve the web site with. Again – it’s not all that different from when a grocery store owner thinks “wow – 10 customers this last week has asked me for low-fat milk. Perhaps I should start to carry that product here”.

I have no problem with both of the above scenarios – they fall into what I call good service and help enhance my online experience. Another problem is that I generally dislike when legal stuff comes in the way for the best technical solution to a problem. Laws should describe the concept of what they are outlawing – not specific technical architectures such as cookies…But before I digress any further into political territory I’ll get right back on track.

Many ASP.NET developers rely on the Session State mechanism to store user relevant data within a visit, that can improve the user experience – for instance with personalization, prefilled forms, and so on. Unfortunately the Session state relies on a unique session key being stored in a local cookie in order to have a unique way to identify the same visitor throughout a visit. It actually comes with a built-in switch to make it stop using cookies – but unfortunately the solution looks rather ugly – it changes all the URLs on the site to contain a Guid and thereby track the visitor using the Guid. I, for one, am rather fond of clean and pretty friendly urls – so that’s no good. So – I started thinking…Many years ago I worked for a company that built a statistics tool. It was pretty unobtrusive and we didn’t use cookies. Instead we just tracked the source IP – and checked for repeated requests with a 10 minutes time-out. Sure, it wasn’t bullet-proof, but it actually worked surprisingly well. And in those cases where it didn’t work? Well – it was just 1 statistical entry out of many. It’s not like we used it to authorize access to the nuclear football, right?! Now, I thought that if we combine all the static information we get in the HTTP Request like IP, Accept Languages, Accept Types, User Agent and so on, smash it all together and take a fingerprint of it – we might end up with something that can almost be used as a session id. Consider: What are the odds that you’ll get 2 different visitors using the exact same configuration, coming from the exact IP on your site within the 20 minutes default time-out?? Of course it turns out I wasn’t the first to think this thought. In fact the clever people at the Electronic Frontier Foundation (EFF) has for some time been running a little example site that calculates those exact odds – just to prove that Privacy online isn’t solved by simply outlawing cookies.

So – I decided to put the thoughts into code. The code consist of 2 parts. First part is an extension method for the HttpRequest class, called “GetUniqueFingerprint()” which will return a MD5 Hash fingerprint.

using System;

using System.Collections.Generic;

using System.Linq;

using System.Web;

using System.Text;

using System.Security.Cryptography;

namespace AllanTech.NoCookie

{

publicstaticclass NoCookies

{

staticprivatestring GetMd5Sum(string s)

{

Encoder enc = System.Text.Encoding.Unicode.GetEncoder();

byte[] text = newbyte[s.Length * 2];

enc.GetBytes(s.ToCharArray(), 0, s.Length, text, 0, true);

MD5 md5 = new MD5CryptoServiceProvider();

byte[] result = md5.ComputeHash(text);

StringBuilder sb = new StringBuilder();

for (int i=0; i<result.Length; i++)

{

sb.Append(result[i].ToString("X2"));

}

return sb.ToString();

}

publicstaticstring GetUnqiueFingerprint(this HttpRequest Request)

{

string source=

string.Join(",", Request.AcceptTypes)+";"+

string.Join(",", Request.UserLanguages)+";"+

Request.UserHostAddress+";"+

Request.UserAgent;

return GetMd5Sum(source);

}

}

}

Second part is a replacement for the ASP.NET SessionIDManager. This is the mechanism that uniquely identifies the visitor – either by a cookie or url – and by replacing it we can make it use our new UniqueFingerprint method instead. It’s really simple – just implement the ISessionIDManager and you’re good to go: